Internal combustion engines such as those found in cars and trucks may produce combustion byproducts and/or products of incomplete combustion which may be in the engine exhaust and emitted into the environment. Per emissions regulations, the exhaust may be treated to reduce the concentration of such products and, therefore, reduce pollution. Although spark ignition (i.e., gasoline) engines may use three-way catalytic converters to satisfy emissions regulations, compression ignition (i.e., diesel) engines typically employ two-way catalytic converters which may not efficiently reduce nitrogen oxides (NOx). Accordingly, diesel engines may include selective catalytic reduction (SCR) systems in order to seek reduction in nitrogen oxide concentrations. Improving performance of such systems remains an ongoing area of research and development.
More specifically, for diesel engines, the main pollutants of concern are nitrogen oxides (NOx) and particulate matter (PM). To reduce NOx it is common to employ NOx reduction systems, including SCR systems to convert NOx (NO and NO2 in some fraction) to N2 and other compounds. SCR systems utilize a reductant, typically ammonia, and an SCR catalyst to convert the NOx. Due to the undesirability of handling pure ammonia, many systems utilize an alternate compound such as urea that vaporizes and decomposes to ammonia before entering the SCR catalyst.